The measuring of parameters representing concentration of an analyte in a measured medium, especially the measuring of pH-value, which reflects the concentration of H+ in the measured medium, plays an important role in environmental analytics and in chemical or biochemical methods in the laboratory and in industrial process measurements technology. Analytes include, for example, certain ion types, such as Cl−, Na+, NO3− or NH4+, or other substances, for example even biomolecules, dissolved in the measured medium. Electrochemical analytical methods, such as, for example, voltammetry, amperometry or potentiometry register, as a rule, the analyte activity, from which analyte concentration can be derived. In dilute solutions, to a first approximation, analyte activity can be set equal to analyte concentration.
A special case of activity, or concentration, measurement is the measuring of the pH-value. The pH-value corresponds to the negative base-10 logarithm of the H+-ion activity in the measured medium, which in dilute solutions can be set equal to the H+-ion concentration.
For measuring ion concentrations or pH-value both in the laboratory as well as also in process analytics, frequently potentiometric sensors are used. These include, as a rule, a measuring half-cell with an ion-selective electrode involving, for example, an ion-selective glass-, solid- or polymer membrane. The relative change of the equilibrium Galvani voltage between a measured medium and a potential sensing electrode of the measuring half-cell is, in such case, essentially effected by the activity change predominantly of the kind of ion to be determined. Referenced to a potential of a reference half-cell of essentially constant potential, e.g. a reference electrode of second type, such as the Ag/AgCl-reference electrode, the sought ion concentration or the pH-value of the measured medium can be determined by means of a high-impedance voltmeter with high accuracy and little apparatus complexity. Serving as measurement signal of such a sensor is thus the potential difference between the measuring- and reference half-cells. Ion selective electrodes are described, for example, in “Ion-Selective Electrodes”, J. Koryta and K. Stulik, Cambridge University Press, 1983, Pg. 61 or in “Das Arbeiten mit ionenselektiven Elektroden (Working with Ion-Selective Electrodes)”, K. Cammann, H. Galster, Springer, 1996.
The most well-known ion-selective electrode and that most frequently applied in such potentiometric sensors as a measuring half-cell is the pH-glass electrode. The glass electrode includes, as a rule, a tubular housing, which is closed on one end by a membrane of a pH-sensitive glass and which is filled with an inner electrolyte, for example, a chloride containing, buffer solution, into which a potential sensing element, for example, a chloridized silver wire, extends. In contact with the measured medium, there forms on the glass membrane a measuring half-cell potential dependent on the pH-value. Serving as a reference half-cell, as a rule, is a reference electrode of a second type, for example, a silver/silver chloride—or calomel-electrode, with a liquid junction, for example in the form of a diaphragm, between, on the one hand, a half-cell space containing the reference electrolyte and, on the other hand, the measured medium. The potential difference between the measuring half-cell potential tappable on the potential sensing element of the measuring half-cell and the reference potential of the reference half-cell (the reference potential of the reference half-cell is ideally independent of the pH-value of the measured medium) forms the measurement signal of the measuring transducer and is a direct measure for the H+-ion activity, respectively the pH-value, of the measured medium.
Although such potentiometric sensors enable very precise and reliable measurement results and are well established both in the laboratory—as well as also in process analytics, they have a number of disadvantages. For example, a series of defects or degradation phenomena of the reference electrodes of the second type serving as a reference half-cell can occur to degrade the quality of the measurement. Thus, the potential of such reference half-cells tends, in practice, generally to drift, i.e. to undergo a slow, however, ongoing, change of the reference potential. Moreover, the inner electrolyte of the reference half-cell can escape or dry out. The liquid junction, via which a reference half-cell of the second type is in contact with the measured medium, can become blocked by solids, especially difficultly soluble salts, and electrode poisons can get into the reference half-cell via the liquid junction. Due to the small conductivity of the pH-sensitive glass membrane, it is additionally required to measure the potential difference between the half-cells with very high impedance, a fact which can lead to instabilities in the measuring and to measured value corruptions. Due to the high resistance of the glass of the glass membrane, limits are set on the miniaturization of such sensors. Thus, with lessening of the glass membrane area, the resistance of the measuring half-cell becomes ever greater. There is, therefore, already long the need for alternative, more robust sensor principles, which should preferably work without one of the conventional reference electrodes of second type.
Described in WO 2005/066618 A1 is a sensor for determining an analyte concentration in a measured medium in a bore hole. The sensor includes a working electrode and a counter electrode as well as an external, reference electrode. Bound on the surface of the working electrode are two or more different molecular species R and M, wherein the molecular species M is sensitive to the analyte L to be determined, for example, binds the analyte L, while the molecular species R is insensitive to the analyte L.
The analyte concentration in the measured medium can be ascertained with this sensor by registering a rectangular wave voltammogram, also referred to as a (linear) square wave voltammogram, SWV. Depending on whether the voltage between working electrode and counter electrode is increased or decreased during the registering of the voltammogram, there occurs on the working electrode an oxidation or a reduction of the molecular species R and M. These oxidation- or reduction processes show up in the plots of the electrical current flowing through the working electrode during the registering of the voltammogram as a function of the associated voltage value as (local) electrical current maxima, or (local) electrical current minima, also referred to as electrical current peaks. When, in the following, maxima, minima or extrema are discussed, unless indicated otherwise, local maxima, minima or extrema are meant.
If present on the working electrode are, respectively, a molecular species R and a molecular species M, there results, assuming that the voltage range of the voltammogram is selected appropriately broadly, respectively a first extremum associated with the molecular species R and a second extremum associated with the molecular species M. While the position of the extremum associated with the analyte sensitive species M changes as a function of the analyte concentration in the surrounding measured medium, the position of the extremum associated with the analyte-insensitive species R is independent of the analyte concentration of the measured medium. The extremum associated with the species R can, thus, serve as an additional, internal reference, so that measurement uncertainties due to degeneration effects of the external reference electrode can be recognized and/or prevented.
Similarly embodied sensors are also known from WO 2005/085825 A1 and WO 2008/154409 A1.
Disadvantageous in the case of such sensors is that the processes on the counter electrode in the described measurements are not defined. This can lead to undesired reactions with the analyte, for example, to gas evolution as a result of water decomposition. Added to this is the fact that, in the case of voltammetric measurements, frequently the oxidizing charge-flows are not equal to the reductive charge-flows on the working electrode. This means that the sensor can change in the course of its operation.